Yes, I think writers can and have created really alien lifeforms. The aliens in Octavia Butler's Xenogenesis trilogy, Robert Forward's neutron star dwellers in Dragon's Egg, and many others come to mind.Read the whole interview for her take on Star Trek-type humanoid aliens, death, environmental balance, and her what she is writing now.
That said, it isn't easy to create really alien-seeming aliens. There are plenty of books with aliens that are just humans in funny suits in a lot of books. Sometimes that's all the story needs. More often it's not.
For me, I find the easiest way to create a really alien-seeming culture is to start with an animal and ecological model. For example, the Tendu and the rainforest, and the harsel and the ocean. I try to avoid mammalian animals, because warm and furry is too familiar, and to anthropomorphize. In The Color of Distance, I based the Tendu on tree frogs. The harsels are based on whale sharks, cold blooded, water-breathing filter feeders. I also try to give my aliens a very different reproductive biology than humans. I find that it gives them very different drives and motives.
Tags:science fiction, Amy Thomson, aliens, ecology